Formation of voids while injection molding and extruding profiles of thermoplastic composites into thick sections and variable thickness parts has been observed for many years. Voids will be formed because the melted composite adjacent the wall of the mold or on the outside of the extrusion solidifies first and permits the inner hot material to shrink towards the outside leaving bubbles in the middle. The concentration of voids in one particular section of a part creates stress and can lead to premature part failure.
In the past several years, Petrarch Systems Inc. of Bristol, Pa. has developed a new class of melt processable thermoplastics which contain silicone interpenetrating polymer networks (IPN's). These compositions are sometimes referred to as semi-interpenetrating polymer networks (pseudo- or semi- IPN's) since only the silicone component is vulcanized or cross-linked in the thermoplastic matrix. The interpenetrating polymer network is formed by vulcanizing a hydride functional silicone group and a polymer containing at least one unsaturated group, preferably a vinyl group, during normal thermoplastic melt processing. See U.S. Pat. No. 4,500,688 and U.S. Pat. No. 4,714,739 of Barry C. Arkles.
The pseudo- or semi-IPN's have the advantages of exhibiting surface and di-electric properties which approach those of silicones and mechanical properties which approach those of the thermoplastic matrix. In many cases, properties such as wear, lubricity and heat and abrasion resistance of the thermoplastics are improved. While this technology was originally developed in polyurethane systems, it has been extended to other thermoplastic elastomers and engineering thermoplastics. For example, U.S. Pat. No. 4,714,739 of Petrarch Systems Inc. discloses that polyamide (nylon) and polyester engineering thermoplastics have been formulated into materials which appear to be particularly well suited for gear and bearing applications, with the heat distortion temperature of nylon 6,6 being increased by the incorporation of 5 wt % silicone IPN.
However, the major uses of silicone IPN's have still been in the modification of lower modulus thermoplastics, particularly elastomers. To applicant's knowledge, the silicone IPN's have not been used or recognized to control void formation of high modulus thermoplastic materials, i.e., thermoplastic materials which in the unfilled state have a flexural modulus greater than 90,000 psi, as measured by ASTM D790.